Contact terminal with doped coating

ABSTRACT

The invention relates to a contact terminal to be used for electrical purposes which contact terminal contains a metallic substrate with good conductivity and coated with a metallic element. In accordance with the invention the coating is doped with at least one additive in order to improve the electrical stability of the coating.

This invention relates to an electrical contact terminal having dopedadditives in the coating material of the coating in order to improve thefunctional performance and reliability.

Tin-coated copper-base alloys are commonly used in electrical contactterminals due to a low price and acceptable reliability for manyapplications. Tin-coated electric contacts are also used for separablecontacts of plug-in-type with limited number of insertion and withdrawalcycles, for instance printed circuit board contacts and pin-socketcontacts.

The main deterioration mechanism for tin-coated contact terminals arefretting caused by mechanical vibration or thermal induced movement.Fretting causes continuous oxidation of the contact area andsubsequently reduction of the available conducting area with an increaseof the contact resistance as a consequence. When almost all of thecontact area is covered by oxide it will result in a steep increase ofthe contact resistance, and in practice, failure will occur. Increasedcontact load is well known to increase the electrical stability andextend the time to failure. However, it will result in a more expensivemechanical design, and also require an increased insertion force.

The objective of the invention is to improve the performance oftin-coated contact terminals by reducing the negative effects offretting associated with the prior art. The essential features of thepresent invention are enlisted in the appended claims.

In accordance with the present invention an electrical contact terminalhas a substrate made of a metal having good conductivity and thesubstrate is coated with a coating layer containing at least one dopedadditive. Using the coating material with the doped additive theelectric stability of the coating layer is improved. In the preferredembodiment of the invention the substrate is made of copper or copperbased alloy, the coating layer is made of tin and the doped additive isphosphorous. The amount of phosphorus is in the range of 0,05 to 2,0atomic %, advantageously 0,1 to 0,25 atomic % phosphorus.

The idea of the preferred embodiment of the invention is that a limitedamount of phosphorus in the tin will act in three steps. Altogetherthese steps will significantly improve the electrical stability, whilsta low contact load can be maintained. The three steps are the following:

1. The phosphorus will limit the formation of tin-oxide at the interfaceof two sliding surfaces, due to its de-oxidizing properties.

2. The formed tin-phosphorus oxide is more brittle and is easier to wipeoff than pure tin oxide. Hence, a significant lower contact load isneeded to achieve an oxide free contact spot.

3. The initial formed tin-oxide between two surfaces is made conductiveby the phosphorus dope additive.

In one another embodiment of the invention the doped additive is acombination of at least two of the group antimony, zinc and cobalt. Thedoped additive can also be at least one of the elements copper, bismuth,silver, zinc, cobalt and antimony or a combination of these elements.Further, in one embodiment of the invention the substrate in the contactterminal is made of aluminium or aluminium based alloy.

The invention is described in more details referring to the followingdrawings wherein

FIG. 1 illustrates the results of fretting tests using phosphorus as anadditive for the time to reach a contact voltage drop of 10 mV with anormal load of 5 N,

FIG. 2 illustrates the results of fretting tests using phosphorus as anadditive for the contact voltage drop as function of the time.

The present invention using phosphorus as a doped additive in the tincoating was tested in a test bench for fretting tests. The said testbench consists of an electronic controlled shaker and a measurementsystem. Before the fretting tests all contacts were subjected to onelong sliding stroke to wipe off the initial surface layer. During thefretting tests, the contacts were subjected to a current load of 2 A DC,and mechanical oscillations of a frequency of 100 Hz with an amplitudeof 20 micrometer. Normal loads of 5 N and 10N were applied. The testswere interrupted just after the contact voltage had passed 70 mV.

Besides pure tin ten different tin phosphorus alloys were produced bycasting rods. Before the fretting tests all samples were turned toachieve a fresh surface. The contact voltage usually increased slowlyfrom a low level to a point when the increase starts to accelerate andfinally rises steeply above the softening and melting voltage of tin,resulting in an unstable electrical contact.

In order to evaluate the effect of phosphorus for the electricalstability during fretting the results of the fretting tests wereevaluated on the basis of the following aspects:

1) The times to reach a contact voltage drop of 10 mV at a contact loadof 5 N.

2) Contact voltage after 1500 seconds (150.000 fretting cycles) at acontact load of 10 N.

FIG. 1 illustrates the time to reach a contact voltage drop of 10 mVwith a normal load of 5 N. Based on the FIG. 1 tin with 0.1 up to 2atomic % of phosphorus achieved in general a significant increased timeto instability compared with the pure tin samples.

FIG. 2 illustrates the contact voltage drop as a function of testingtime for 0.4 atomic % and 1.6 atomic % of phosphorus compared with puretin when a 10 N normal load is applied. The difference between thephosphorus doped tin samples and the pure tin sample is remarkable. Thelow and stable contact resistances for the tin-phosphorus samples were aresult of the achieved gross welded contact spots. Additionalexperiments indicated that for pure tin at least three times highercontact load (30 N at the present test conditions) is needed to achievea gross welded contact spot.

1. Contact terminal to be used for electrical purposes which contactterminal comprises a metallic substrate with good conductivity andcoated with a metallic element, the coating being made of tin that isdoped with at least one additive that improves electrical stability ofthe coating.
 2. Contact terminal according to claim 1, wherein theamount of a phosphorus dope additive in the coating is between 0.05-2.0atomic %.
 3. Contact terminal according to claim 1 wherein the amount ofphosphorus dope additive in the coating is between 0.1-0.2 atomic %. 4.Contact terminal according to claim 1, wherein the dopes additive is atleast one of the elements copper, bismuth, silver, zinc, cobalt andantimony or a combination of the elements.
 5. Contact terminal accordingto claim 1, wherein the dope additive is a combination of at least twoselected from the group consisting of antimony, zinc and cobalt. 6.Contact terminal according to claim 1 wherein the substrate in thecontact terminal is made of copper based alloy.
 7. Contact terminalaccording to claim 1 wherein the substrate in the contact terminal ismade of aluminum based alloy.